Physics Research Conference

Thermal detectors based on superconducting transition-edge sensors (TES) have become, by some metrics, the most sensitive photon detectors over more than 8 orders of magnitude of energy: from 100 GHz microwaves to 100 keV gamma rays. To achieve this level of performance, it has been necessary to understand these devices from the perspectives of material science, condensed-matter physics, quantum mechanics, and nonequilibrium thermodynamics. I will discuss the physics of these devices, including their particularly interesting nonequilibrium properties. While it is a resistive sensor, the TES operates in a nonequilibrium regime, and it does not exhibit equilibrium Johnson-Nyquist noise. Instead, its noise is consistent with the Stratonovich nonequilibrium fluctuation-dissipation relations, which are based on an extension of equilibrium thermodynamics using the time-reversibility of Markov processes. I will discuss techniques to read out large arrays of these devices with either superconducting microwave resonators or multiplexed superconducting quantum interference devices. As an illustration, I will describe the recent application of these devices to problems in nuclear security, including nuclear safeguards, non-proliferation, and forensics.